Hot rolled steel sheets with tensile strength of 440 MPa or less have been conventionally used for parts such as frames and chassis of vehicles and trucks. However, automotive steel sheets having higher strength and smaller thickness are recently required in order to improve automobile crashworthiness and to preserve the global environment. Studies have been, therefore, launched for the use of high-strength hot rolled steel sheets with 590 MPa tensile strength, 780 MPa tensile strength, and 980 MPa or higher tensile strength.
Automobile parts frequently have complicated shapes imparted by press forming, and thus the material steel sheet requires high strength and excellent workability. On the other hand, the thickness reduction of steel sheet entails ensuring the corrosion resistance of car bodies. From this viewpoint, there has been a demand for coated steel sheets obtained by imparting corrosion resistance to the material steel sheet. In particular, galvannealed steel sheets have been desired which have excellent corrosion resistance and weldability and may be produced at low cost.
High-strength hot rolled steel sheets with excellent workability, high-strength galvanized steel sheets and the manufacturing methods thereof have been proposed. For example, Patent Literature 1 discloses a high-strength steel sheet with tensile strength of not less than 590 MPa and excellent workability, and a method for manufacturing such steel sheets. Specifically, a steel including, by mass %, C: 0.02 to 0.06%, Si≦0.3%, Mn: 0.5 to 2.0%, P≦0.06%, S≦0.005%, Al≦0.06%, N≦0.006%, Mo: 0.05 to 0.5%, Ti: 0.03 to 0.14%, and the balance being substantially Fe, is smelted and hot rolled under conditions in which the finishing delivery temperature is not less than 880° C. and the coiling temperature is not less than 570° C. The thus-obtained steel sheet has a microstructure which is substantially composed of a ferrite single phase and in which carbide precipitates containing titanium and molybdenum with average particle size of less than 10 nm are dispersed.
In a method disclosed in Patent Literature 2, a steel including, by mass %, C: 0.01 to 0.1%, Si≦0.3%, Mn: 0.2 to 2.0%, P≦0.04%, S≦0.02%, Al≦0.1%, N≦0.006%, Ti: 0.03 to 0.2%, one or both of Mo≦0.5% and W≦1.0%, and the balance being Fe and inevitable impurities is smelted and hot rolled in an austenite single phase region and coiled at not less than 550° C., thereby producing a ferrite single phase hot rolled steel sheet, and the steel sheet is further descaled and directly subjected to galvanization. The disclosed high-strength galvanized hot rolled steel sheet satisfies 4.8C+4.2Si+0.4Mn+2Ti≦2.5 in terms of mass %, has a microstructure with a ferrite area ratio of 98% or more, and less than 10 nm precipitates are dispersed in the microstructure, the precipitates containing titanium and one or both of molybdenum and tungsten while satisfying the atomic ratio (Mo+W)/(Ti+Mo+W)≧0.2.
Because Patent Literatures 1 and 2 involve the precipitation of fine carbides containing titanium and other elements such as molybdenum in ferrite, the coiling after the hot rolling is preferably performed at a coiling temperature (hereinafter, also written as CT) of 550° C. or above. When a hot rolled steel sheet containing such elements as silicon and manganese which are more prone to oxidation than iron (hereinafter, also written as easily oxidizable elements) is coiled at such a high CT, internal oxides containing such easily oxidizable elements are formed in the surface layer of the base steel sheet. As a result, the Zn—Fe alloying reaction is excessively promoted during the subsequent galvanization and alloying treatments, causing a deterioration in coating adhesion. Further, hot rolled steel sheets have a larger thickness than cold rolled steel sheets. Because of this fact, the coating film of galvanized hot rolled steel sheets undergoes larger compression strain during forming. Thus, coating adhesion is deteriorated to a further extent.
For example, an approach for improving coating adhesion is described in Patent Literature 3, which discloses a galvannealed steel sheet having excellent powdering resistance and excellent low-temperature chipping resistance. This galvannealed steel sheet is obtained by subjecting a galvanized steel sheet to alloying treatment and thereafter to mechanical extension control or pickling so as to produce 10 or more cracks per 1 mm on the surface of coating layer. Although coating adhesion is improved by the technique disclosed in Patent Literature 3, the fact that the coating layer has a high density of cracks gives rise to a concern that corrosion resistance may be deteriorated.
Patent Literature 4 discloses a technique which improves coating adhesion by obtaining smaller crystal grain boundaries on the steel sheet surface by performing grinding the base steel sheet before reduction annealing in a galvanizing line. However, the technique does not consider corrosion resistance and is also problematic in that the addition of a grinding facility increases the cost.
Patent Literature 5 discloses a galvannealed steel sheet with excellent corrosion resistance and coating adhesion which has a Mg-enriched layer at the surface layer of the Zn coating layer and also has a Ni—Fe—Al—Zn layer at the interface between the coating layer and the base steel sheet. However, an increase in cost is of concern because the production entails a facility for performing Ni precoating before reduction annealing and also because expensive elements such as magnesium and nickel are added. Further, the bath composition used in the coating treatment is different from that of a zinc coating bath used in a common galvanizing line. Thus, the implementation of the disclosed technique is difficult due to facility cost and operation control.
It is known that steel sheets having tensile strength of 980 MPa or above suffer hydrogen brittleness by the entry of hydrogen into the steel. Thus, there is urgency in developing a technique capable of improving hydrogen brittleness resistance of high-strength hot rolled steel sheets with 980 MPa or higher tensile strength.
However, any of the techniques disclosed in Patent Literatures 1 to 5 does not consider how to improve hydrogen brittleness resistance when the techniques are applied to high-strength steel sheets with 980 MPa or higher tensile strength.